A flaw was found in 389-ds-base. The get_ldapmessage_controls_ext() function in the LDAP server does not enforce an upper bound on the number of controls per LDAP message. A remote, unauthenticated attacker can send a specially crafted LDAP request containing hundreds of thousands of minimal controls within the default maximum BER message size (2 MB), causing excessive CPU consumption and heap allocation on the server. Under concurrent exploitation, this leads to significant latency degradation, worker thread starvation, or out-of-memory termination, resulting in a denial of service.
Restrict network access to the LDAP port (389/tcp, 636/tcp) to trusted networks only using firewall rules or network ACLs. This prevents untrusted remote attackers from reaching the vulnerable code path.
Optionally, lower the nsslapd-maxbersize configuration parameter to reduce the maximum BER message size accepted by the server. Note that this caps bytes, not the number of controls, and does not fully eliminate the amplification. Setting it too low may impact legitimate LDAP operations with large payloads.